Most industrial sieving machines include some means of applying a primary vibratory movement to the sieving screen in order to facilitate product movement through the screen and also to create a flow of material over the screen surface. This ensures maximum utilisation of the active screening area and that oversized product can be transported to an outlet to be removed. The primary vibratory movement is often a combination of horizontal and vertical reciprocating motion which may typically be applied to the frame carrying the sieve mesh or screen in a variety of ways, such as by rotating out-of-balance weights, or a direct drive by a rigid crank or cam system.
A problem with sieving machines is blinding of the screen, particularly when sieving damp or sticky materials. Blinding is a significant problem in the industrial sieving of certain powders and also in the straining of liquids. To overcome the blinding problem secondary vibrations, preferably flexural, have been applied to the screen, for example by impacts from deblinding discs or the application of high and ultrasonic frequencies (see for example EP-A-0369572).
Typical ultrasonic frequencies are above 20 kHz, and typical amplitudes of the ultrasonic vibration supplied to the mesh are a few (1-10) microns. However, ultrasonic energy is quickly dissipated in the screen, making it difficult to excite a large screen area ultrasonically. Extended resonators to increase the distribution of ultrasonic energy over the screen are disclosed in EP 0652810. However, for large sieve areas, multiple transducers are still normally required.
It is also known to use guide members located above the screen to improve the flow of material to be sieved over the surface of the screen. For example, scroll-shaped guide members are used with circular sieves to ensure material to be sieved moves progressively from the centre of the screen, where it is first delivered, outwards in a generally spiral path, covering nearly all regions of the screen surface before reaching the outlet for oversize particles at or near the screen periphery. This increases the residence time over the screen, to maximise the opportunity for fines to pass through the screen. Other guide member shapes and arrangements are used for different sieve designs, in each case to improve material flow over the screen to increase the time for undersize to separate from oversize.